Add FastAPI Agent Blueprint

[Mr-DooSun]

Summary

• add FastAPI Agent Blueprint to the repository index
• allow repository index entries without a Sourcegraph repository id
• update Sourcegraph upserts so a manually seeded URL can be reconciled if Sourcegraph later indexes it

Notes

FastAPI Agent Blueprint is a FastAPI backend blueprint for AI-agent-oriented services. I noticed this project is generated from Sourcegraph, and Sourcegraph does not currently return this repository. I kept this as a draft PR in case manually seeded entries are acceptable. If the index should remain strictly generated, closing this is completely fine.

Checks

• python -m ruff check --verbose --format=github .
• python -m mypy --show-error-codes --pretty --show-column-numbers --show-error-context .
• python -m black --check --verbose .
• python -m pyproject_fmt --check --indent=4 .
• python -m pytest -v -s --failed-first --cov=app --cov-report=xml --cov-branch
• pnpm lint
• pnpm prettier:lint
• pnpm next build
• pnpm next export